High strength, low carbon, dual phase steel rods and wires and process for making same

ABSTRACT

A high strength, high ductility, low carbon, dual phase steel wire, bar or rod and process for making the same is provided. The steel wire, bar or rod is produced by cold drawing to the desired diameter in a single multipass operation a low carbon steel composition characterized by a duplex microstructure consisting essentially of a strong second phase dispersed in a soft ferrite matrix with a microstructure and morphology having sufficient cold formability to allow reductions in cross-sectional area of up to about 99.9%. Tensile strengths of at least 120 ksi to over 400 ksi may be obtained.

The present invention is directed to a process for making high-strength,high-ductility, low-carbon steel wires, bars and rods by cold drawingdual-phase steels. Here, the term "dual-phase steels" refers to a classof steels which are processed by continuous annealing, bach annealing,or conventional hot rolling to obtain a ferrite matrix with a dispersedsecond phase such as martensite, bainite and/or retained austenite. Thesecond phase is controlled to be a strong, tough and deformable phaseunlike the hard, non-deformable carbide phase found in pearlitic rodsand wires. It must be suitably dispersed and in sufficient volumefraction i.e. greater than 10%, to provide a substantial contribution tothe strength in the as-heat-treated condition and to increase thework-hardening rate during wire drawing. Various heat treatment pathscan be used to develop the dual-phase microstructure and the morphologydepends on the particular heat treatment employed. A preferred heattreatment is the intermediate quench method i.e. austenitize and quenchto 100% martensite prior to annealing in the two phase α+γ field andquenching to a ferrite martensite structure. The invention is furtherdirected to the high-structure, high ductility steel wires, bars androds produced by the process of the present invention.

Steel wire has many known uses, such as for making cables, chains, andsprings. It is also used to make steel belts and bead wire for tires,and steel strands are included in multistrand electrical wire to improvethe tensile strength of the wire. In these applications, the diameterrequirements range from 0.005 inch to more than 0.25 inch with strengthrequirements ranging from 250 ksi to as much as 400 ksi in the smallerdiameters. In all of these applications, it is important to provide asteel wire having a high tensile strength and good ductility at therequired diameter.

The oldest and most common method of producing high strength, highductility wire is by patenting a near eutectoid composition pearliticsteel. However, this process is complex and expensive. A furtherdisadvantage of the patenting method is an inherent limitation in themaximum wire diameter that can be produced at a given strength level.

There is a need for steel wire and rods having higher tensile strengthand higher ductility than steel wire and rods produced by the knownmethods, as well as a more economical method for producing high strengthsteel wire and rods. The present invention would replace theconventional method of patenting pearlitic steel to produce wwire with aprocess whereby an alloy of relatively simple composition is cold drawninto wire or rods in a single multipass operation, i.e., withoutintermediate annealing or patenting heat treatments. Elimination of thepatenting heat treatments in the production of high strength steel wireshould lower the cost of producing high strength steel wire, especiallyin light of the present fuel situation.

The cold drawing process requires a low alloy steel composition with amicrostructure and morphology which provides high initial strength, highductility, rapid work hardening, and good cold formability. The steelshould be capable of being cold drawn, without intermediate anneals orpatenting heat treatments, to the desired diameter, tensile strength,and ductility.

A specific group of steels with a chemical composition specificallydeveloped to impart higher mechanical property values is known in theart as high-strength, low-alloy (HSLA) steel. These steels containcarbon as a strengthening element in an amount reasonably consistentwith weldability and ductility. Various levels and types of alloycarbide formers are added to achieve the mechanical properties whichcharacterize these steels. However, the high tensile strength and highductility needed in many applications for steel wire and rods do notseem to be attainable using HSLA steels.

The factors governing the properties of low carbon steels are primarilyits carbon content and microstructure, and secondarily the residualalloy. Commonly, low carbon steels contain silicon, manganese, or acombination of silicon and manganese. In addition, carbide formingelements such as, vanadium, chromium, niobium, molybdenum may be added.

Low carbon, dual-phase microstructured steels characterized by a strongsecond phase dispersed in a soft ferrite matrix show potential forsatisfying the tensile strength, ductility, flexibility and diameterrequirements of high strength steel wire. Furthermore, they havepotential for achieving a level of cold formability which allows colddrawing without patenting or intermediate heating. In particular, a lowcarbon, duplex ferrite-martensite steel, disclosed in U.S. Pat. No.4,067,756 issued Jan. 10, 1978, is of interest in the present inventionbecause it has high strength, high ductility characteristics and iscomposed of inexpensive elements. However, as generally fabricated, ithas a tensile strength of about 120 ksi which is much lower than thetensile strength required for most applications of high strength steelwire. The process of the present invention is directed to producing ahigh-strength steel wire having a tensile strength of at least about 120ksi. A preferred tensile strength range is 120 ksi to 390 ksi, butstrengths above 400 ksi may be achieved.

It is therefore an object of the present invention to provide animproved process for making high strength, high ductility steel wiresand rods and to produce steel wires or rods with increased tensilestrength, ductility, and flexibility at the desired diameter.

Another object of the present invention is to provide a process formaking high strength, high ductility steel wires or rods comprising thestep of cold drawing a dual-phase steel composition to the requiredstrength and ductility without intermediate anneals or patenting heattreatments, thereby providing complete flexibility in choosing the wirediameter.

It is another object of the present invention to provide a process formaking high strength, high ductility steel wires or rods that eliminatesthe intermediate patenting step used in the present process for makingpearlitic steel wire, thereby reducing the complexity, cost, and energyconsumption of the process for making high strength steel wires androds.

A further object of the present invention is to provide a process formaking high strength steel wires and rods which is versatile, allowingfor a wide range of diameters, strength, and ductility properties in thefinal steel wire or rod based on the choice of the initial duplexmicrostructure and manipulation of the microstructure throughappropriate thermal processing.

A further object of the present invention is to provide high strength,high ductility steel wires or rods which have a tensile strength atleast about 120 ksi.

Additional objects and advantages of the present invention will becomeevident from the following description taken in conjunction with theaccompanying drawings.

In general, the present invention is directed to high strength, highductility, low carbon steel wires or rods and the process for making thesame. The process involves cold drawing a low carbon dual-phase steel tothe desired diameter in a single multipass operation. The steel ischaracterized by a duplex microstructure consisting essentially of astrong second phase dispersed in a soft ferrite matrix and amicrostructure and morphology having sufficient cold formability toallow reductions in cross-sectional area of up to about 99.9%.

One preferred embodiment of the invention is a high strength, highductility, low carbon steel rod or wire produced from a steelcomposition characterized by an appropriate duplex ferrite-martensitemicrostructure, for example as shown in FIG. 1, and the process formaking the same. The process involves cold drawing the duplexferrite-martensite steel to the desired diameter in a single multipassoperation. In high strength steels with a duplex ferrite-martensitemicrostructure, the strong, deformable second phase consistspredominately of martensite but may contain bainite and retainedaustenite. The strong second phase is dispersed in a soft ductileferrite matrix; the martensite provides the strength in the compositewhereas the ferrite provides the ductility.

FIG. 1 is an optical micrograph showing a typical low carbon dual-phaseferrite-martensite microstructure prior to cold drawing.

FIG. 2 is a transmission electron micrograph of dislocated lathmartensite which comprises the strong second phase in a dual-phase steelaccording to the present invention.

FIG. 3 is a graph exemplifying a typical comparison between a colddrawing schedule for duplex microstructure steel wire according to thepresent invention and a drawing schedule for pearlitic steel wireaccording to a patenting method.

According to the present invention, the high strength, high ductilitysteel wires or rods are produced by a process whereby a low carbon steelcomposition, characterized by a duplex microstructure consistingessentially of a strong second phase dispersed in a soft ferrite matrix,is cold drawn to the desired diameter in a single multipass operation.The starting steel composition prior to the cold drawing should possessa duplex microstructure and a morphology which are sufficient to providea level of cold formability allowing reductions in cross-sectional areaof up to 99.9% during cold drawing.

The process of the invention provides an advantage over known processesin that it eliminates the intermediate heat treatments or patentingsteps used in the known process for making pearlitic steel wire, andthereby reduces the complexity, cost, and energy consumption of theprocess. Furthermore, a wider range of rod and wire diameter sizes canbe produced by the process of the invention than in the patentingmethod. In the patenting method, there is an inherent limitation in themaximum wire diameter that can be produced at a given strength level.

Referring to FIG. 3, the differences between the process of the presentinvention and the patenting process are shown. The solid lineillustrates the cold drawing schedule of a low carbon duplex steel wireaccording to the present invention and the tensile strengths which canbe achieved at different diameters. The broken lines indicate thedrawing schedule of a pearlitic steel wire according to the patentingmethod, including the intermediate heat treatments. In the drawing ofthe pearlitic steel wire, the intermediate heat treatments are necessaryin order to achieve the greater tensile strength that the process of thepresent invention can achieve at various diameters. These intermediateheat treatments increase the complexity and expense of the process formaking high-strength steel wire. The process according to the presentinvention does not involve intermediate heat treatments and thusprovides a significant improvement over the known process.

The process of the present invention can produce steel wires and rodswith a wide range of tensile strength, ductility, and diameter. Thefinal properties of the steel wire or rod at a given diameter aredetermined by a combination of the initial microstructure, theproperties of the starting steel and the amount of subsequent reductionin cross-sectional area during the cold drawing process. Since themicrostructure of the steel is easily manipulated through appropriatethermal processing, the properties of the drawn wire can be tailored tomatch the required specifications of the desired application. The choiceof alloying elements such as silicon, aluminum, manganese, and carbideforming elements, such as, molybdenum, niobium, vanadium, and the like,is determined by the microstructure and properties desired. Thus, a widerange of alloys, including many simple and inexpensive alloys, can beused as long as they can be heat treated to the desired dual-phasemicrostructure.

One preferred duplex microstructure is the ferrite-martensitemicrostructure. Another preferred microstructure is the duplexferrite-bainite microstructure. In both caeses, the strong second phase,either martensite or bainite, is dispersed in a soft, ductile ferritematrix.

In one preferred embodiment of the process of the present invention thestarting steel composition consists essentially of iron, from about 0.05to 0.15 weight % carbon, and from about 1.0 to 3.0 weight % silicon. Inanother preferred embodiment the starting steel composition consistsessentially of iron, from about 0.05 to 0.15 weight percent carbon, fromabout 1-3 weight percent silicon, and from about 0.05 to 0.15 weightpercent vanadium. In both preferred embodiments, the steel compositionis thermally treated form a duplex ferrite-martensite microstructure ina fibrous morphology. Briefly, the preferred process comprises the stepsof austenitizing the steel composition, quenching the steel compositionto transform the austenite to substantially 100% martensite, heating theresulting steel composition to an annealing temperature for a timesufficient to provide the desired ratio of austenite and ferrite, quickquenching the austenite ferrite composition to transform the austeniteto martensite, and cold drawing the resulting dual-phase steel which ischaracterized by a duplex ferrite-martensite microstructure in a fibrousmorphology to the desired diameter in a single multipass operation.

More specifically, the starting steel composition is heated to atemperature, T₁, above the critical temperature at which austeniteforms. The temperature range for T₁, is from about 1050° C. to 1170° C.The composition is held at that temperature for a period of timesufficient to substantially and completely austenitize the steel. Theresulting composition is quenched in order to transform the austenite tosubstantially 100% martensite. The composition is then reheated to anannealing temperature, T₂, in the two phase (α+γ) range. The α+γtemperature range is from about 800° C. to 1000° C. The composition isheld at this temperature for a period of time sufficient to transformthe martensitic steel composition to the desired volume ratio of ferriteand austenite. Upon final quenching, the austenite transforms tomartensite, resulting in a strong second phase of martensite dispersedin a soft or ductile ferrite matrix.

The steel composition at this point is characterized by a uniquemicrostructure which is a fine, isotropic, acicular martensite in aductile ferrite matrix. The microstructure results due to thecombination of the double heat treatment and the presence of silicon inthe above-specified amount. The unique microstructure maximizes thepotential ductility of the soft phase ferrite and also fully exploitsthe strong martensite phase as a load carrying constituent in the duplexmicrostructure. It is the microstructure as well as the morphology ofthe steel composition that enables the steel to be cold drawn to thedesired wire or rod diameter in a single multipass operation.

Any dual-phase steel may be used in the process of the present inventionas long as a duplex microstructure and morphology can be produced havingsufficient cold formability to allow reductions in cross-sectional areaof up to about 99.9% when the composition is cold drawn. In particular,dual-phase ferrite-martensite steels have a greater continuous yieldingbehavior, higher ultimate tensile strength, and better ductility thancommercial high strength low alloy steels, including microalloyed,fine-grain steels. Furthermore, the high tensile to yield ratio and highstrain hardening rate in ferrite-martensite dual-phase steel providesexcellent cold formability.

The exact temperature, T₁, to which the steel composition is heated inthe first austenization step is not critical as long as it is above thetemperature at which complete austenization occurs. The exacttemperature, T₂, in the second heating step where the composition istransformed to the two phases of ferrite and austenite depends upon thedesired volume ratio of ferrite and austenite, which in turn dependsupon the desired volume ratio of ferrite-to-martensite. In general, thedesired volume ratio of ferrite and martensite depends upon the ultimateproperties desired for the steel wire or rod. Generally, the 10-40volume percent of martensite in the ferrite-martensite microstructurewill allow the steel composition to be cold drawn to diametersrepresenting up to 99.9% reductions in cross sectional area and willstill result in steel wires and rods having a tensile strength at leastabout 120 ksi. Usually, tensile strengths in the range of 120 ksi to 390ksi are obtained, but 400 ksi and above may also be obtained.

The following examples will illustrate the process of the invention moreclearly, the resulting properties of the steel wires and rods producedby the process, as well as the flexibility of the process in allowing achoice of alloys, tensile strengths, ductility, and diameters.

EXAMPLE 1

A high strength, high ductility steel wire was made to satisfy therequirements for bead wire used in the manufacture of automobile tires.The bead wire requires a tensile strength of 270 ksi with 5% elongation,and a proportional limit of 216 ksi. The bead wire should be about 0.037inch in diameter with sufficient ductility to pass a torsion testrequiring 58 axial twists in an 8 inch length. A 0.220 inch diametersteel rod having a composition consisting essentially of iron, 0.1weight percent carbon, 2 weight percent silicon, and 0.1 weight percentvanadium was austenitized and rapidly quenched to yield a substantially100% martensitic composition. The rod was then reheated to a temperatureof 950° C. in the two phase α+γ range and rapidly quenched to produce aduplex ferrite-martensite microstructure of approximately 30 volumepercent martensite and 70 volume percent ferrite. The needle-like,acicular character of the ferrite-martensite microstructure is shown inthe optical micrograph in FIG. 1. The heat treated rod was then colddrawn through lubricated conical dies down to a diameter of 0.037 inchin 8 passes of approximately 36% reduction in area per pass. After ashort stress relief anneal at 425° C. similar to the current practice,an ultimate tensile strength of 276 ksi was achieved, thus satisfyingthe tensile strength requirement of bead wire. The ductility of thesteel wire was sufficient to satisfy the twist test requirement.

EXAMPLE 2

A steel rod consisting essentially of iron, 0.1 weight percent carbon,and 2.0 weight percent silicon was hot rolled to a diameter of 0.25inch. The rod was then heated to a temperature of about 1150° C. forabout 30 minutes to austenitize the composition. The steel was thenquenched in iced brine to transfom the austenite to substantially 100%martensite. The rod was then rapidly reheated to a temperature of 950°C. in order to convert the structure to qpproximately 70% ferrite and30% austenite. The steel rod was then quenched in iced brine to convertthe austenite to martensite. Finally, the rod was cold drawn to adiameter of 0.030 inch where its tensile strength was 357 ksi, and alsodrawn to a diameter of 0.024 inch where its tensile strength was 360ksi. Continued cold drawing may achieve tensile strengths to 400 ksi orhigher.

What is claimed is:
 1. A process for making high strength, highductility steel wire and rods comprising the steps of:heating a steelcomposition consisting essentially of iron, from about 0.05 to 0.15 wt.% carbon, and from about 1.0 to 3.0 wt. % silicon to a temperature T₁for a period of time sufficient to substantially completely austenitizedsaid steel; quenching the resulting austenitized steel composition totransform said austenite to 100% martensite; heating the resultingmartensitic steel composition to a temperature T₂ in the two phase α+γrange for a period of time sufficient to transform said martensiticsteel composition to a volume ratio of ferrite and austenite such thatsubsequent quenching results in a microstructure having about 10 to 40volume percent martensite and about 60 to 90 volume percent ferrite;quenching the resulting ferrite-autenite steel composition to transformthe austenite to martensite; and cold drawing the resulting steelcomposition, said steel composition characterized by a duplexferrite-martensite microstructure having about 10 to 40 volume percentmartensite and about 60 to 90 volume percent ferrite, saidmicrostructure imparting to said steel composition cold formabilityproperties permitting said steel composition to be cold drawn to adiameter representing up to about a 99.9% reduction in cross-sectionalarea and imparting to the resulting cold drawn steel product tensilestrength properties of up to 400 ksi.
 2. A process according to claim 1wherein the cold drawing step is accomplished in a single multipass. 3.A process according to claim 1 wherein said steel composition consistsessentially of iron, about 0.1 wt. % carbon, and about 2 wt. % silicon.4. A process according to claim 1 wherein said steel compositioncontains from about 0.05 to 0.15 wt. % vanadium.
 5. A process accordingto claim 4 wherein said vanadium content is about 0.1 wt. percent.
 6. Aprocess according to claim 1 or 4 wherein T₁ is in the range from about1050° C. to 1170° C. and T₂ is in the range from about 800° C. to 1000°C.
 7. A process according to claim 1 or 4 wherein T₁ is about 1150° C.8. A process according to claim 1 or 4 wherein T₂ is about 950° C. andthe resulting microstructure contains about 30 volume percentmartensite.
 9. A high strength, high ductility, low carbon steel productproduced by the process of claim
 7. 10. A steel product according toclaim 9 wherein said product is steel wire.
 11. A steel productaccording to claim 9 wherein said product is steel bar or rod.
 12. Ahigh strength, high ductility, low carbon steel product according toclaim 10 or 11 wherein said carbon content is about 0.1 wt. %, saidsilicon content is about 2 wt. %, said duplex ferrite-martensitemicrostructure comprises about 30 volume percent martensite, and saidtensile strength is from about 357 ksi to about 360 ksi after about99.9% reduction in cross-sectional area.
 13. A high strength, highductilty, low carbon steel product according to claim 10 or 11 whereinthe steel composition contains from about 0.05 to 0.15 wt. % vanadium.14. A high strength, high ductility, low carbon steel product accordingto claim 13 wherein said carbon content is about 0.1 wt. %, said siliconcontent is about 2.0 wt. %, said vanadium content is about 0.1 wt. %,said duplex ferrite-martensite microstructure comprises 30 volumepercent martensite, and said tensile strength is about 276 ksi afterabout 97% reduction in cross-sectional area.